The GNU Hurd is the GNU project's replacement for the Unix kernel. It is a collection of servers that run on the Mach microkernel to implement file systems, network protocols, file access control, and other features that are implemented by the Unix kernel or similar kernels (such as Linux).

Since day one of the GNU project, Hurd was supposed to be its kernel - as we all know, of course, it turned out Linux provided a far better kernel with a much faster pace of development, and it's been used as the de-facto GNU kernel ever since. Those with an appreciation for history will love the lingering, mildly dismissive tone of "...such as Linux".